Passive immunization of mice. A Mice (n = 5) received mAb 2D1 intraperitoneally 1 h prior to the challenge. Mice were then challenged with a 2 × LD50 dose of 8.42 × 109 CFU of G. parasuis by intraperitoneal inoculation. mAb ApxIV was used as a negative control, and positive serum against G. parasuis-5 was used as the positive control. Mice were observed for 7 days after the challenge, and the survival rate of mice in each group was compared. B Blood bacterial loads of ICR mice after lethal challenge were determined in serial dilutions of 20 µL of blood obtained from the tail vein at 12 h, 24 h, 48 h, and 72 h postinfection in a sterile environment. After being diluted with sterile PBS, the blood was applied to TSA solid medium and incubated at 37 °C for 24 h, and the bacterial load in blood was calculated.